This invention relates to a solid-state image sensing device and a method of driving a solid-state image sensing device.
When a solid-state image sensing device used as an image pickup device is applied to an electronic camera, it is necessary to secure a sufficiently large dynamic range. Because the dynamic range of solid-state image sensing device is significantly narrower than that of silver film.
So, Japanese patent application laid-open No. 8-9260 (1996) discloses a technique that the dynamic range is enlarged by varying a voltage of substrate within an imaging period to switch the amount of accumulable charge of a photodiode. This conventional technique is explained below.
FIG. 1 is a plan view showing cell part in, e.g. a CCD type solid-state image sensing device. The cell part is composed of a photoelectric conversion section 101, a vertical charge transfer electrode 102, a first charge transfer electrode 105 and a second charge transfer electrode 106.
FIG. 2 is a cross sectional view cut along the line I-Ixe2x80x2 in FIG. 1. As shown, the cell part is composed of an N type semiconductor substrate 107, a Pxe2x88x92 type semiconductor substrate 108, an N type semiconductor substrate 109, a P+ type semiconductor substrate 110, a first charge transfer electrode 105 formed with first-layer polysilicon 111, a second charge transfer electrode 106 formed with second-layer polysilicon 112. aluminum film 113 serving as shade film, insulating film 114, and cover insulating film 115.
FIG. 3 is a characteristic diagram showing the electronic potential of photoelectric conversion section.
First, in order to reset unnecessary electric charge before accumulating charge into photodiode, a substrate voltage VHsub is applied to the Nxe2x88x92 type semiconductor substrate 107, depleting completely the N type semiconductor substrate 109 composing the photoelectric conversion section 101 and the Pxe2x88x92 type semiconductor substrate 108 with a low concentration formed just hereunder, moving all the unnecessary charge to the Nxe2x88x92 type semiconductor substrate 107.
Such a structure is generally called xe2x80x9cvertical overflow drain structure (vertical OFD)xe2x80x9d (reference: J. of Institute of Television Engineers of Japan, Vol. 37, No. 10 (1983), pp. 782-787.
Subsequently, a substrate voltage Vbsub (hereinafter referred to as xe2x80x98substrate voltagexe2x80x99) is applied to the Nxe2x88x92 type semiconductor substrate 107, the photoelectric conversion section 101 starts accumulating a signal charge according to amount of incident light. Hereupon, by adjusting the substrate voltage arbitrarily, such excessive charge that cannot be accumulated in the photoelectric conversion section 101 is moved into the Nxe2x88x92 type semiconductor substrate 107 using the vertical OFD structure, thereby the controlling of amount of accumulable charge is conducted.
Using this technique, the solid-state image sensing device is controlled so that the amount of accumulable charge in solid-state image sensing device is switched sequentially from a first amount of accumulable charge (Qsat(1)xe2x89xa00) to a second amount of accumulable charge (Qsat(2)xe2x89xa00, Qsat(1) less than Qsat(2)) within one imaging period.
By changing the substrate voltage applied to OFD (overflow drain) of solid-state image sensing device at time t(1) within imaging period to conduct this operation, the substrate voltage is controlled so that the amount of accumulable charge in solid-state image sensing device is kept Qsat(1) from the beginning to time t(1) within one imaging period and, after time t(1), switched into Qsat(2).
FIG. 4 shows the relationship (solid line) between an amount of accumulable charge within one imaging period and charge accumulation time, in a solid-state image sensing device having such a function. FIG. 5 shows the relationship (solid line) between an amount of accumulable charge within one imaging period and an amount of light.
Dotted lines in FIGS. 4 and 5 indicate characteristics in the case that the amount of accumulable charge does not vary within one imaging period.
As shown in FIGS. 4 and 5, comparing with the case that the amount of accumulable charge does not vary, the dynamic range can be enhanced.
Namely, by providing a means for switching the amount of accumulable charge in solid-state image sensing device sequentially from the first amount of accumulable charge (Qsat(1) xe2x89xa00) to the second amount of accumulable charge (Qsat(2)xe2x89xa00, Qsat(1) less than Qsat(2)) within one imaging period, the dynamic range can be enhanced.
However, in the conventional solid-state image sensing device, when t(1) is set within one imaging period and Qsat(1) and Qsat(2) are set only under the condition of Qsat(1) less than Qsat(2), the dynamic range may not be improved sufficiently. Also, there may occur a case chat the dynamic range is improved little more than the case that the amount of accumulable charge does not vary. The reason is as explained below.
FIGS. 6A to 6C show the relationship between charge accumulation time where t(1) varies among t(1a), t(1b) and t(1c) and the amount of accumulable charge. FIGS. 7A to 7C show the relationship between an amount of incident light and the amount of accumulable charge. Meanwhile, t(1a) less than t(1b) less than t(1c) is satisfied, t(1b) is the middle point of one imaging period, and 2Qsat(1)=Qsat(2) is satisfied. Dotted lines indicate characteristics in the case that the maximum amount of accumulable charge is constant.
As seen from FIGS. 6A to 6C and 7A to 7C, in case of t(1c). the dynamic range is enhanced comparing with the case that the maximum amount of accumulable charge is constant. However, in case of t(1a) and t(1b), the dynamic range is not enhanced comparing with the case that the maximum amount of accumulable charge is constant.
This is because t(1), Qsat(1) and Qsat(2) are determined only under the condition of Qsat(1) Qsat(2). At this condition, the dynamic range cannot be improved surely comparing with the case that the amount of accumulable charge is constant. Further, the circuit is complicated comparing with the case that the amount of accumulable charge is constant.
Japanese patent application laid-open No. 1-253960 (1989) discloses a solid-state image sensing device where the saturation amount of signal transfer is made larger than the amount of signal charge in saturation of light-receiving element. However, it does not describe about that the amount of accumulable charge is varied in the form of multiple stages within one imaging period.
Also, Japanese patent application laid-open No. 5-22728 (1993) discloses a technique that the amount of accumulable charge is varied according to the gain of amplification circuit corresponding to solid-state image sensing device and the gain of white-balance adjustment circuit. However, it does not describe about that the amount of accumulable charge is varied in the form of multiple stages within one imaging period.
Further, Japanese patent application laid-open No. 10-150183 (1998) discloses a solid-state image sensing device equipped with a drive system that reduces an OFD bias to solid-state image sensing element when reading an amount of charge. However, it does not describe about that the amount of accumulable charge is varied in the form of multiple stages within one imaging period.
Accordingly, it is an object of the invention to provide a solid-state image sensing device and a method of driving a solid-state image sensing device that the dynamic range is improved effectively.
It is an object of the invention to provide a solid-state image sensing device and a method of driving a solid-state image sensing device that even when the photoelectric conversion efficiency is varied with amount of light, an image that does not give uncomfortable feeling to eyes can be produced.
According to the invention, a solid-state image sensing device, comprises:
a plurality of sensing means arrayed in the form of a matrix;
a charge accumulation means that is connected to the sensing means and accumulates a charge generated at the sensing means;
an accumulable charge adjusting means that adjusts the amount of accumulable charge of the charge accumulation means; and
a control means that controls the accumulable charge adjusting means;
wherein the control means controls the amount of accumulable charge to vary continuously or discontinuously in time series within one imaging period and within a given amount of accumulable charge of the control means.
According to another aspect of the invention, a method of driving a solid-state image sensing device which comprises a plurality of sensing means arrayed in the form of a matrix,
a charge accumulation means that is connected to the sensing means and accumulates a charge generated at the sensing means, an accumulable charge adjusting means that adjusts the amount of accumulable charge of the charge accumulation means, and a control means that controls the accumulable charge adjusting means, comprising the step of:
controlling the amount of accumulable charge to vary continuously or discontinuously in time series within one imaging period and within a given amount of accumulable charge of the sensing means, by the control means.
According to another aspect of the invention, a recording medium that stores a program to make a computer conduct a method of driving a solid-state image sensing device which comprises a plurality of sensing means arrayed in the form of a matrix, a charge accumulation means that is connected to the sensing means and accumulates a charge generated at the sensing means, an accumulable charge adjusting means that adjusts the amount of accumulable charge of the charge accumulation means, and a control means that controls the accumulable charge adjusting means, comprising the step of:
controlling the amount of accumulable charge to vary continuously or discontinuously in time series within one imaging period and within a given amount of accumulable charge of the sensing means, by the control means.